Re: Vacuum pump, what is it? Reply #20 – April 11, 2020, 12:56:42 am Quote from: MT Ted – April 10, 2020, 09:49:31 pmWhat part of Montana are you headed? In my coach, theres an auxiliary pump switch by the dock light and over ride switches. I have to turn that little pump on when shes cold. It pumps hot coolant to the front. I turn it on for a minute or two and the heater works fine. Without that little pump it will stay cold. We're headed to Hungry Horse for the summer. On the drawing we have, it shows one of relays in isolator board area to be for the aux pump but that it's only applicable to the U300 series. Quote Selected